Radio Communication Device and Radio Communication Method

ABSTRACT

A radio base station  100  judges a receiving state of a radio signal in conformity with the orthogonal frequency division multiplexing scheme, and changes a symbol string configuration of a symbol string defined by a size in a frequency axis direction and in a time axis direction, on the basis of the judged receiving state.

TECHNICAL FIELD

The present invention relates to a radio communication device and aradio communication method for transmitting and receiving radio signalsincluding symbol strings having a predetermined size in a frequency axisdirection and in a time axis direction, by using the orthogonalfrequency division multiplexing scheme.

BACKGROUND ART

Recently, in a radio communication system such as WiMAX (worldwideinteroperability for microwave access), for example, the orthogonalfrequency division multiplexing (OFDM) scheme has been used in order toutilize radio resources more efficiently. In addition, adaptive arraycontrol by an adaptive array antenna has also been introduced in suchradio communication system.

In the adaptive array control, an array weight used to form a beam of aradio signal to be transmitted to a communication destination iscalculated based on “known information” included in a radio signalreceived from the communication destination (a mobile station, forexample).

In addition, in the OFDM scheme, a symbol string in which multiplesymbols are arranged in a time axis direction and a frequency axisdirection (subcarrier direction) is used. Generally, the symbol stringincludes: preamble symbols used in symbol synchronization andcalculation of an array weight; and data symbols used in transmission ofuser data.

For a case where the adaptive array control is introduced in a radiocommunication system to which the OFDM scheme is applied as describedabove, there has been known a method in which the preamble symbol partand the data symbol part of known information used to calculate an arrayweight is changed in order to calculate the array weight more accurately(Patent Document 1, for example).

Patent Document 1: JP-A 2002-185375 (pages 6 to 7, FIG. 2)

DISCLOSURE OF THE INVENTION

Meanwhile, in the radio communication system described above, aso-called orthogonal frequency division multiple access (OFDMA) has alsobeen implemented in which a predetermined number of subcarriers areassigned to each user in order to enable multiple users to performcommunications simultaneously.

However, if the number of subcarriers assigned to each user is small,namely, if a frequency band assigned to each user is narrow, there is aproblem that an antenna combined gain that can be provided by an arrayweight obtained using an algorithm such as minimum mean squared error(MMSE) is far from a theoretical value.

Specifically, when the frequency band assigned to each user is narrow,the number of preamble symbols arranged in the frequency axis directionis small. Accordingly, the number of preamble symbols necessary forobtaining the antenna combined gain close to the theoretical valuecannot be ensured.

Then, it may be considered to arrange more preamble symbols in the timeaxis direction in order to obtain the antenna combined gain close to thetheoretical value. However, if more preamble symbols are arranged in thetime axis direction, the proportion of the preamble symbols included ina symbol string becomes higher than that of the data symbols. Thisraises another problem that transmission efficiency of the data symbolsis degraded.

Hence, the present invention has been made in light of suchcircumstances, and an object of the present invention is to provide aradio communication device and a radio communication method that canperform adaptive array control in which a desired antenna combined gaincan be obtained irrespective of a receiving state of a radio signal,while suppressing degradation of transmission efficiency of datasymbols.

The present invention has the following characteristics to solve theproblems described above. The first characteristic of the presentinvention is a radio communication device (radio base station 100) thatreceives a radio signal (uplink radio signal RS_(UP)) from acommunication destination (radio communication terminal 200), by usingthe orthogonal frequency division multiplexing scheme, the radio signalincluding a symbol string (a symbol string S, for example) having apredetermined size (2×2 configuration, for example) in a frequency axisdirection (frequency axis direction D_(F)) and in a time axis direction(time axis direction D_(T)), and performs control of a radio signal(downlink radio signal RS_(DOWN)) to be transmitted to the communicationdestination, on the basis of the symbol string included in the receivedradio signal, the radio communication device comprising: a receivingstate judging unit (receiving unit judging unit 107) configured to judgea receiving state (for example, reception SNR) of the radio signal; anda symbol string configuration changing unit (symbol string configurationchanging unit 109) configured to change a symbol string configuration(2×4 configuration, for example) of the symbol string defined by thesize in the frequency axis direction and in the time axis direction, onthe basis of the receiving state judged by the receiving state judgingunit.

According to such radio communication device, the symbol stringconfiguration is changed, as appropriate, to the symbol stringconfiguration of the symbol string defined by the size in the frequencyaxis direction and in the time axis direction, on the basis of areceiving state of the radio signal following the orthogonal frequencydivision multiplexing (OFDM) scheme.

In other words, according to such radio communication device, when thereceiving state of the radio signal is good, a symbol stringconfiguration having a large size in the frequency axis direction can beused. Meanwhile, if the receiving state of the radio signal is poor, asymbol string configuration having large size in the time axis directioncan be used.

Consider a case where a basic size of the data symbol part in the timeaxis direction is fixed, such as in WiMAX. Here, when the receivingstate of the radio signal is good, a greater number of preamble symbolscan be arranged in the frequency axis direction by increasing the sizeof the symbol string in the frequency axis direction, the preamblesymbols being necessary to obtain an antenna combined gain close to atheoretical value by control of a radio signal transmitted to acommunication destination such as adaptive array control. Consequently,the number of preamble symbols arranged in the time axis direction isreduced, thereby improving transmission efficiency of the data symbols.

Moreover, if the receiving state of the radio signal is poor, thepreamble symbols necessary to obtain the antenna combined gain close tothe theoretical value by the adaptive array control can be arranged byincreasing the size in the time axis direction while avoiding effect offrequency selective fading, although the transmission efficiency of thedata symbols degrades.

The second characteristic of the present invention is a characteristicaccording to the first characteristic of the present invention, thesecond characteristic further comprising a symbol configurationnotification unit (symbol string configuration notification unit 111)configured to notify the communication destination of the symbol stringconfiguration changed by the symbol string configuration changing unit.

The third characteristic of the present invention is a characteristicaccording to the first or second characteristic of the presentinvention, wherein the symbol string configuration includes a referencesymbol string configuration (4×2 configuration) and a narrowband symbolstring configuration (2×2 configuration) having a size in the frequencyaxis direction smaller than the reference symbol string configuration,and the symbol string configuration changing unit changes the symbolstring configuration from the reference symbol string configuration tothe narrowband symbol string configuration, when the receiving statejudging unit judges that the receiving state satisfies a predeterminedcondition.

The fourth characteristic of the present invention is a characteristicaccording to the first or second characteristic of the presentinvention, wherein the wideband symbol string configuration includes areference symbol string configuration (4×2 configuration) and a widebandsymbol string configuration (8×1 configuration) having a size in thefrequency axis direction larger than the reference symbol stringconfiguration, and the symbol string configuration changing unit changesthe symbol string configuration from the reference symbol stringconfiguration to the wideband symbol string configuration, when thereceiving state judging unit judges that the receiving state satisfies apredetermined condition.

The fifth characteristic of the present invention is a characteristicaccording to any one of the first to third characteristic of the presentinvention, wherein the symbol string configuration includes a referencesymbol string configuration (4×2 configuration), a first narrowbandsymbol string configuration (2×2 configuration) and a second narrowbandsymbol string configuration (2×4 configuration), the first narrowbandsymbol string configuration having a size in the frequency axisdirection smaller than the reference symbol string configuration, thesecond narrow band symbol string configuration having a in the time axisdirection larger than the first narrowband symbol string configuration,and the symbol string configuration changing unit changes the symbolstring configuration to the first narrowband symbol string configurationinstead of the reference symbol string configuration when the receivingstate judging unit judges that the receiving state does not satisfy apredetermined condition, and changes the symbol string configurationfrom the first narrowband symbol string configuration to the secondnarrowband symbol string configuration when the receiving state judgingunit judges that the receiving state does not satisfy the predeterminedcondition even in the first narrowband symbol string configuration.

The sixth characteristic of the present invention is a characteristicaccording to the third characteristic of the present invention, whereinthe radio communication device receives a radio signal including thesymbol string and performs adaptive array control of the radio signaltransmitted to the communication destination on the basis of the symbolstring included in the received radio signal, the symbol string includesa preamble symbol (preamble symbol P) used in calculation of an arrayweight, and the symbol string configuration changing unit changes thereference symbol string configuration and the narrowband symbol stringconfiguration to symbol string configurations having a same number ofpreamble symbols.

The seventh characteristic of the present invention is a characteristicaccording to the fourth characteristic of the present invention, whereinthe radio communication device receives a radio signal including thesymbol string and performs adaptive array control of the radio signaltransmitted to the communication destination on the basis of the symbolstring included in the received radio signal, the symbol string includesa preamble symbols (preamble symbols 2) used in calculation of an arrayweight, and the symbol string configuration changing unit changes thereference symbol string configuration and the wideband symbol stringconfiguration to symbol string configurations having a same number ofpreamble symbols.

The eighth characteristic of the present invention is a characteristicaccording to the first to seventh characteristic of the presentinvention, wherein the symbol string includes a preamble symbol(preamble symbol P) used in calculation of an array weight, and thesymbol string configuration changing unit changes a symbol stringconfiguration of the preamble symbol defined by a size in the frequencyaxis direction and in the time axis direction.

The ninth characteristic of the present invention is a radiocommunication method in which a radio signal including a symbol stringhaving a predetermined size in the frequency axis direction and in thetime axis direction is received from a communication destination byusing the orthogonal frequency division multiplexing scheme, and anadaptive array control of a radio signal to be transmitted to thecommunication destination is performed on the basis of the symbol stringincluded in the received radio signal, the radio communication methodcomprising the steps of: judging a receiving state of the radio signal;and changing a symbol string configuration of the symbol string definedby the size in frequency axis direction and in the time axis directionon the basis of the judged receiving state.

The tenth characteristic of the present invention is a characteristicaccording to the tenth characteristic of the present invention furthercomprising the step of notification the communication destination of thesymbol string configuration changed in the step of changing the symbolstring configuration.

The eleventh characteristic of the present invention is a characteristicaccording to the ninth or tenth characteristic of the present invention,wherein a reference symbol string configuration and a narrowband symbolstring configuration are included in the symbol string configuration,the narrowband symbol string configuration having a size in thefrequency axis direction smaller than the reference symbol stringconfiguration, and, in the step of changing the symbol stringconfiguration the symbol string configuration is changed from thereference symbol string configuration to the narrowband symbol stringconfiguration, when it is judged that the receiving state satisfies apredetermined condition.

The twelfth characteristic of the present invention is a characteristicaccording to the ninth or tenth characteristic of the present invention,wherein a reference symbol string configuration and a wideband symbolstring configuration are included in the symbol string configuration,the wideband symbol configuration having a size in the frequency axisdirection larger than the reference symbol string configuration, and, inthe step of changing the symbol string configuration, the symbol stringconfiguration is changed from the reference symbol string configurationto the wideband symbol string configuration, when it is judged that thereceiving state satisfies a predetermined condition.

The thirteenth characteristic of the present invention a characteristicaccording to the eleventh characteristic of the present invention,wherein the symbol string includes preamble symbols used in calculationof an array weight, and in the step of changing the symbol stringconfiguration, the reference symbol string configuration and thenarrowband symbol string configuration are changed into symbol stringconfigurations including a same number of preamble symbols.

The fourteenth characteristic of the present invention is acharacteristic according to the twelfth characteristic of the presentinvention, wherein the symbol string includes a preamble symbol used incalculation of an array weight, and, in the step of changing the symbolstring configuration, the reference symbol string configuration and thewideband symbol string configuration are changed into symbol stringconfigurations including a same number of preamble symbols.

A fifteenth characteristic of the present invention is a characteristicaccording to any one of the ninth to fourteenth characteristic of thepresent invention, wherein the symbol string includes a preamble symbolused in calculation of array weight, and, in the step of changing thesymbol string configuration, the symbol string configuration of thepreamble symbols defined by the size in the frequency axis direction andin the time axis direction is changed.

According to the characteristics of the present invention, a radiocommunication device and a radio communication method can be providedthat can execute adaptive array control in which a desired antennacombined gain can be obtained irrespective of a receiving state of aradio signal, while suppressing degradation of transmission efficiencyof data symbols.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is an overall schematic configuration diagram of a radiocommunication system according to an embodiment of the presentinvention.

FIG. 2 is a functional block configuration diagram of a radiocommunication device (radio base station) according to the embodiment ofthe present invention.

FIG. 3 is a functional block configuration diagram of a radiocommunication device (radio communication terminal) according to theembodiment of the present invention.

FIG. 4 is an operation flowchart of the radio communication device(radio base station) according to the embodiment of the presentinvention.

FIG. 5 is an operation flowchart of the radio communication device(radio communication terminal) according to the embodiment of thepresent invention.

FIG. 6 is a view showing one example of a symbol string according to theembodiment of the present invention.

FIG. 7 is a diagram showing an example of a symbol string according tothe embodiment of the present invention.

FIG. 8 is a diagram showing an example of a symbol string according tothe embodiment of the present invention.

FIG. 9 is a diagram showing an example of a symbol string according tothe embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinafter. Notethat, in the following description of the drawings, same or similarparts are denoted with same or similar reference numerals. It should benoted, however, that the drawings are schematic and proportions or thelike of each of the dimensions differ from actual dimensions.

Hence, specific dimensions or the like should be determined byconsidering the following description. It is also needless to say thatrespective drawings may include parts that have different dimensionalrelations or proportions from each other.

(Overall Schematic Configuration of Radio Communication System)

FIG. 1 is an overall schematic configuration diagram of a radiocommunication system according to the embodiment. As shown in FIG. 1,the radio communication system according to the embodiment consists of aradio base station 100 and a radio communication terminal 200. Notethat, the number of the radio base station and the radio communicationterminal is not limited to the number shown in FIG. 1.

In the radio base station 100 and the radio communication terminal 200,the Orthogonal Frequency Division Multiplexing (OFDM) scheme is used.Specifically, the radio base station 100 and the radio communicationterminal 200 are a radio communication device complying with WiMAX(worldwide interoperability for microwave access). In WiMax, so-calledOrthogonal Frequency Division Multiple Access (OFDMA) is used whichassigns a predetermined number of subcarriers to each user.

In addition, the radio base station 100 performs adaptive array controlof a downlink radio signal RS_(DOWN) to be transmitted to the radiocommunication terminal 200 (communication destination), on the basis ofa symbol string included in an uplink radio signal RS_(UP) received fromthe radio communication terminal 200.

In the embodiment, the radio communication terminal 200 is a card typeradio communication terminal that is to be attached to a notebook-typepersonal computer, PDA (not shown) or the like.

(Functional Block Configuration)

Next, configurations of the radio base station 100 and the radiocommunication terminal 200 will be described. Note that, in thefollowing, parts related to the present invention will be mainlydescribed. Therefore, it should be noted that the radio base station 100and the radio communication terminal 200 may include a logical block(such as a power supply unit) that is essential to implement functionsas the device but is not shown or omitted of description.

(1) Radio Base Station 100

FIG. 2 is a functional block configuration diagram of the radio basestation 100. As shown in FIG. 2, the radio base station 100 includes aradio communication unit 101, an array controller 103, a radio signalprocessor 105, a receiving state judging unit 107, a symbol stringconfiguration changing unit 109, a symbol string configurationnotification unit 111, and a baseband processor 113.

The radio communication unit 101 transmits and receives a radio signalin accordance with the OFDM scheme. Here, FIG. 6 shows one example of asymbol string included in an uplink radio signal RS_(UP) that the radiocommunication unit 101 receives from the radio communication terminal200.

As shown in FIG. 6, a symbol string S has a predetermined size in thefrequency axis direction D_(F) and in the time axis direction D_(T). Thesymbol string S is formed of a preamble symbol P and a data symbol D.Moreover, the data symbol D includes a predetermined number of pilotsymbols PL.

The frequency axis direction D_(F) is defined by the number ofsubcarriers, i.e., a bandwidth. In the embodiment, 9 carriers (one linerepresents one carrier in the diagram) are defined as one unit (1 bin).Moreover, the time axis direction D_(T) is defined by the number ofOFDMA symbols.

In the embodiment, the data symbol D is configured of 18 carriers,namely 2 bins, in the frequency axis direction D_(F), and 3 symbols inthe time axis direction D_(T), as a basic size. More than one datasymbols D of the basic size can be arranged along the frequency axisdirection D_(F).

Moreover, in the symbol string S, the preamble symbol P is configured of18 carriers, namely, 2 bins, in the frequency axis direction D_(F), and2 symbols (2×2 configuration) in the time axis direction D_(T) as abasic size. In the embodiment, a symbol string configuration isidentified based on the configuration of the preamble symbol P. Namely,the symbol string S is expressed as a 2×2 configuration. The preamblesymbol P is used in calculation of an array weight.

The array controller 103 performs adaptive array control of a downlinkradio signal RS_(DOWN) to be transmitted to the radio communicationterminal 200, on the basis of a symbol string S included in an uplinkradio signal RS_(UP) received from the radio communication terminal 200.

Specifically, the array controller 103 calculates an array weight to beapplied to the downlink radio signal RS_(DOWN) on the basis of thepreamble symbol P (see FIG. 6). The array controller 103 also controls abeam pattern of the downlink radio signal RS_(DOWN) to be transmitted bythe radio communication unit 101 on the basis of the calculated arrayweight.

The radio signal processor 105 performs processing related to thedownlink radio signal RS_(DOWN) transmitted by the radio communicationunit 101 and an uplink radio signal RS_(UP) received by the radiocommunication unit 101. In the embodiment, in particular, the radiosignal processor 105 can change a symbol string configuration of asymbol string S included in the downlink radio signal RS_(DOWN).

The receiving state judging unit 107 judges a receiving state of theuplink radio signal RS_(UP) received by the radio communication unit101. Specifically, the receiving state judging unit 107 estimates asignal-to-noise ratio (reception SNR) of the uplink radio signalRS_(UP). Specifically, the receiving state judging unit 107 calculatescorrelation of the preamble symbol P and the data symbol D, on the basisof the preamble symbol P included in the uplink radio signal RS_(UP),and estimates the reception SNR of the uplink radio signal RS_(UP)(subcarriers). In addition, in the radio communication system accordingto the embodiment, transmission power, a phase, time (transmissioncycle) and the like of the uplink radio signal RS_(UP) is adjusted sothat the reception SNR becomes largest. Namely, a target SNR is set inthe radio communication system according to the embodiment, the targetSNR having a control evaluation value being a reception SNR that will benecessary depending on a modulation scheme, and control of the uplinkradio signal RS_(UP) is performed so as to maintain the target SNR.

The symbol string changing unit 109 changes a symbol stringconfiguration of a symbol string S on the basis of the receiving statejudged by the receiving state judging unit 107. In the embodiment, a 4×2configuration shown in FIG. 7, a 2×4 configuration shown in FIG. 8, andan 8×1 configuration shown in FIG. 9 can be used, in addition to the 2×2configuration shown in FIG. 6.

In the embodiment, the 4×2 configuration shown in FIG. 7 is referred toas a reference symbol string configuration. In addition, the 2×2configuration shown in FIG. 6 and the 2×4 configuration shown in FIG. 8are referred to as a narrowband symbol string configuration which issmaller in size in the frequency axis direction D_(F) compared to thereference symbol string configuration. In addition, the 8×1configuration shown in FIG. 9 is referred to as a wideband symbol stringconfiguration which is larger in size in the frequency axis directionD_(F) compared to the reference symbol string configuration.

Namely, in the embodiment, the symbol string configuration changing unit109 changes a symbol string configuration of the entire symbol string Son the basis of a symbol string configuration of the preamble symbol P(4×2 configuration, for example).

When it is judged that the receiving state of the uplink radio signalRS_(UP) satisfies a predetermined condition, specifically, when thereception SNR measured by the receiving state judging unit 107 fellbelow a predetermined threshold, the symbol string configurationchanging unit 109 changes symbol string configuration from the referencesymbol string configuration (4×2 configuration) to the narrowband symbolstring configuration (2×2 configuration).

Moreover, when the reception SNR measured by the receiving state judgingunit 107 fell under the predetermined threshold, the symbol stringconfiguration changing unit 109 may change the symbol stringconfiguration from the 2×2 configuration (first narrowband symbol stringconfiguration) to the 2×4 configuration (second narrowband symbol stringconfiguration). In other words, the symbol string configuration changingunit 109 can change the symbol string configuration to a symbol stringconfiguration including a same number of preamble symbols, in thereference symbol string configuration and the narrowband symbol stringconfiguration.

In addition, when the reception SNR measured by the receiving statejudging unit 107 exceeds a predetermined threshold, the symbol stringconfiguration changing unit 109 changes the symbol string configurationfrom the reference symbol string configuration (4×2 configuration) tothe wideband symbol string configuration (8×1 configuration). Namely,the symbol string configuration changing unit 109 can change symbolstring configuration to a symbol string configuration including a samenumber of preamble symbols, in the reference symbol string configurationand the wideband symbol string configuration.

The symbol string configuration notification unit 111 notifies the radiocommunication terminal 200 of the symbol string configuration changed bythe symbol string configuration changing unit 109. Specifically, thesymbol string configuration notification unit 111 transmits a symbolstring configuration notification indicating the symbol stringconfiguration changed by the symbol string configuration changing unit109, to the radio communication terminal 200. Note that, the symbolstring configuration notification is included in a downlink radio signalRS_(DOWN) and transmitted to the radio communication terminal 200.

The baseband processor 113 is connected to the radio signal processor105. The baseband processor 113 transmits a baseband signal such as userdata, control data and the like to the radio signal processor 105. Thebaseband processor 113 also demodulates a radio signal received from theradio signal processor 105 into a baseband signal.

(2) Radio Communication Terminal 200

FIG. 3 is a functional block configuration diagram of the radiocommunication terminal 200. As shown in FIG. 2, the radio communicationterminal 200 includes a radio communication unit 201, a radio signalprocessor 203, a symbol string configuration receiver 205, a symbolstring configuration changing unit 207, and a baseband processor 209.Note that, in the following, a description of functional blocks thatperform similar processing to the radio base station 100 described abovewill be omitted, as appropriate.

The radio communication unit 201 performs processing almost similar tothe radio communication unit 101. Moreover, the radio signal processor203 performs processing almost similar to the radio signal processor105.

The symbol string configuration receiver 205 receives the symbol stringconfiguration notification transmitted by the radio base station 100.The symbol string configuration receiver 205 notifies the symbol stringconfiguration changing unit 207 of the symbol string configuration ofthe symbol string S included in the uplink radio signal RS_(UP), on thebasis of the received symbol string configuration notification.

The symbol string configuration changing unit 207 changes the symbolstring configuration of the symbol string included in the uplink radiosignal RS_(UP) on the basis of the symbol string configuration notifiedby the symbol string configuration receiver 205. Specifically, thesymbol string configuration changing unit 207 changes the symbol stringconfiguration to any of the 4×2 configuration, the 2×2 configuration,the 2×4 configuration and a 8×1 configuration.

The baseband processor 209 performs processing almost similar to thebaseband processor 113.

(Operation of Radio Communication System)

The operation of the radio communication system described above will bedescribed hereinafter. Specifically, described is: (1) the operation ofthe radio base station 100 changing the symbol string configuration ofthe symbol string S on the basis of the receiving state of the uplinkradio signal RS_(UP); and (2) the operation of the radio communicationterminal 200 setting the symbol string configuration of the symbolstring S included in the uplink radio signal RS_(UP), on the basis ofthe symbol string configuration notification transmitted by the radiobase station 100.

(1) Radio Base Station 100

FIG. 4 is an operation flow diagram of the radio base station 100. Asshown in FIG. 4, in step S10, the radio base station 100 estimatesreception SNR of an uplink radio signal RS_(UP) received from the radiocommunication terminal 200. Specifically, the radio base station 100calculates correlation of a preamble symbol P and a data symbol D on thebasis of the preamble symbol P included in the uplink radio RS_(UP)signal, and estimates the reception SNR of the uplink radio signalRS_(UP) (subcarrier).

In step S20, the radio base station 100 judges whether the estimatedreception SNR satisfies a predetermined condition. Specifically, theradio base station 100 judges whether the estimated reception SNR issmaller than a threshold β, equal to or larger than a threshold α, orlarger than the threshold β and smaller than the threshold α.

If the estimated reception SNR is smaller than the threshold β (in thecase of “RECEPTION SNR<β” in step S20), in step S30A, the radio basestation 100 changes the symbol string configuration of the symbol stringS included in the uplink radio signal RS_(UP) to the 2×4 configuration.

If the estimated reception SNR is equal to or larger than the thresholdα (in the case of “RECEPTION SNR≧α” in step S20), in step S30B, theradio base station 100 changes the symbol string configuration of thesymbol string S included in the uplink radio signal RS_(UP) to the 4×2configuration.

If the estimated reception SNR is larger than the threshold β andsmaller than the threshold α (in the case of “B<RECEPTION SNR<A” in stepS20), in step S30C, the radio base station 100 changes the symbol stringconfiguration of the symbol string included in the uplink radio signalRS_(UP) to the 2×2 configuration.

In step S40, the radio base station 100 stores the symbol stringconfiguration that has been changed in one of the steps of steps S30A toS30C.

In step S50, the radio base station 100 transmits a symbol stringconfiguration notification indicating the changed symbol stringconfiguration to a communication destination, namely, the radiocommunication terminal 200.

As described above, in the embodiment, as a symbol string configuration,the 4×2 configuration (reference symbol string configuration), the 2×2configuration (first narrowband symbol string configuration) which has asize smaller in the frequency axis direction D_(F) than the 4×2configuration, and the 2×4 configuration (second narrowband symbolstring configuration) which has a size larger in the time axis directionD_(T) than the 2×2 configuration are provided.

The radio base station 100, more specifically, the symbol stringconfiguration changing unit 109 changes from the 4×2 configuration thesymbol string configuration to the 2×2 configuration, when the receivingstate judging unit 107 judges that the receiving state of the uplinkradio signal RS_(UP) does not satisfy the predetermined condition. Inaddition, when it is judged that the receiving state of the uplink radiosignal RS_(UP) does not satisfy the predetermined condition even in the2×2 configuration, the symbol string configuration changing unit 109 canchange the symbol string configuration from the 2×2 configuration to the2×4 configuration.

In the operation flow shown in FIG. 4, the radio base station 100changes the symbol string configuration to one of the 4×2 configuration,the 2×2 configuration, or the 2×4 configuration. However, the symbolstring configuration may be also changed to the 8×1 configuration (seeFIG. 9). In this case, as a condition for changing the symbol stringconfiguration to the 8×1 configuration, the radio base station 100 usesreception SNR that is better than the threshold a used in judgingwhether or not to change the symbol string configuration to the 4×2configuration.

(2) Radio Communication Terminal 200

FIG. 5 is an operation flowchart of the radio communication terminal200. As shown in FIG. 5, in step S110, the radio communication terminal200 judges whether or not a symbol string configuration notification hasbeen received from the currently communicating radio base station 100.

If the symbol string configuration notification has been received (YESin step S110), in step S120, the radio communication terminal 200 setsthe symbol string configuration of the symbol string S included in theuplink radio signal RS_(UP), on the basis of the received symbol stringconfiguration notification.

In step S130, the radio communication terminal 200 transmits, to theradio base station 100, the uplink radio signal RS_(UP) including thesymbol string S having the set symbol string configuration.

EFFECTS AND ADVANTAGES

As described above, in the embodiment, a basic size of the data symbol Dis 18 carriers in the frequency axis direction D_(F) (2 bins) and 3symbols in the time axis direction D_(T). Namely, the basic size of thedata symbol D in the frequency axis direction D_(F) and in the time axisdirection D_(T) is fixed.

In the embodiment, when reception SNR of an uplink radio signal RS_(UP)is good (in the case of the reception SNR≧α, for example), theconfiguration of the symbol string S included in the uplink radio signalRS_(UP) is changed to a symbol string configuration in which the size ofthe symbol string S is large in the frequency axis direction D_(F) (4×2configuration, for example). Accordingly, a greater number of preamblesymbols P, which are necessary to obtain an antenna combined gain closeto a theoretical value by adaptive array control, can be arranged in thefrequency axis direction D_(F). Thus, the number of the preamble symbolsP to be arranged in the time axis direction D_(T) is reduced, therebyimproving transmission efficiency of the data symbol D. Furthermore, asthe size in the time axis direction D_(T) becomes smaller, effect offading due to multipath is suppressed, and uniform communication qualitycan be ensured even if the radio communication terminal 200 moves athigh speed.

Moreover, when the reception SNR of the uplink radio signal RS_(UP) ispoor (in the case of Reception SNR<β, for example), the configuration ofthe symbol string S included in the uplink radio signal RS_(UP) ischanged to a symbol string configuration in which the size of the symbolstring S is large in the time axis direction D_(T) (2×4 configuration,for example). Accordingly, the preamble symbol P, which is necessary toobtain the antenna combined gain close to the theoretical value by theadaptive array control, can be arranged while avoiding the effect of thefrequency selective fading, although the transmission efficiency of thedata symbol D degrades.

Other Embodiments

As described above, although the present invention is disclosed throughone embodiment of the present invention, it should not be understoodthat dissertation and drawings that form a part of the disclosure limitthe present invention. Various alternative embodiments will be apparentto one skilled in the art from this disclosure.

For example, the embodiment described above takes a form in which, asthe symbol string configuration, size of a preamble symbol P in thefrequency axis direction D_(F) and in the time axis direction D_(T) ischanged depending on the receiving state of the uplink radio signalRS_(UP). However, needless to say, it may also take a form in which thesize of not the preamble symbol P part but the entire symbol string S inthe frequency axis direction D_(F) and in the time axis direction D_(T)is changed.

In addition, the embodiment described above takes a form in which thereception SNR is used as the receiving state of the uplink radio signalRS_(UP). However, as the receiving state of the uplink radio signalRS_(UP), a received power value, timing jitter, or frequency offsetamount of the uplink radio signal RS_(UP) may also be used.

Furthermore, in the embodiment described above, the radio communicationterminal 200 is a card type radio communication terminal. However, theradio communication terminal 200 may take any form other than the cardtype, for example, a mobile phone terminal.

In addition, the embodiment described above takes a form in which theradio communication terminal 200 sets a symbol string configuration of asymbol string S included in an uplink radio signal RS_(UP) on the basisof a symbol string configuration notification received from the radiobase station 100. However, it may take a form in which the radiocommunication terminal 200 estimates the condition of the uplink radiosignal RS_(UP) on the basis of the receiving state of the downlink radiosignal RS_(DOWN) and sets the symbol string configuration of the symbolstring S included in the uplink radio signal RS_(UP).

As described above, of course, the present invention includes variousembodiments that have not been described herein. Therefore, a technicalscope of the present invention is defined only by matters specific tothe invention according to scope of claims appropriate from theabove-described description.

Additionally, the entire content of Japanese Patent Application No.2006-207240 (filed on Jul. 28, 2006) is incorporated herein byreference.

INDUSTRIAL APPLICABILITY

As described above, the radio communication device and the radiocommunication method according to the present invention can performadaptive array control in which a desired antenna combined gain can beobtained irrespective of the receiving state of a radio signal, whilesuppressing degradation of transmission efficiency of data symbols.Accordingly, the radio communication device and the radio communicationmethod according to the present invention are useful in radiocommunications such as mobile communication.

1. A radio communication device that receives a radio signal from acommunication destination by using the orthogonal frequency divisionmultiplexing scheme, the radio signal including a symbol string having apredetermined size in a frequency axis direction and in a time axisdirection, and performs control of a radio signal to be transmitted tothe communication destination, on the basis of the symbol stringincluded in the received radio signal, the radio communication devicecomprising: a receiving state judging unit configured to judge areceiving state of the radio signal; and a symbol string configurationchanging unit configured to change a symbol string configuration of thesymbol string defined by the size in the frequency axis direction and inthe time axis direction, on the basis of the receiving state judged bythe receiving state judging unit.
 2. The radio communication deviceaccording to claim 1, further comprising a symbol configurationnotification unit configured to notify the communication destination ofthe symbol string configuration changed by the symbol stringconfiguration changing unit.
 3. The radio communication device accordingto claim 1 or 2, wherein the symbol string configuration includes areference symbol string configuration, and a narrowband symbol stringconfiguration having a size in the frequency axis direction smaller thanthe reference symbol string configuration, and the symbol stringconfiguration changing unit changes the symbol string configuration fromthe reference symbol string configuration to the narrowband symbolstring configuration, when the receiving state judging unit judges thatthe receiving state satisfies a predetermined condition.
 4. The radiocommunication device according to claim 1 or 2, wherein the symbolstring configuration includes a reference symbol string configuration,and a wideband symbol string configuration having a size in thefrequency axis direction larger than the reference symbol stringconfiguration, and the symbol string configuration changing unit changesthe symbol string configuration from the reference symbol stringconfiguration to the wideband symbol string configuration, when thereceiving state judging unit judges that the receiving state satisfies apredetermined condition.
 5. The radio communication device according toclaim 1 or 2, wherein the symbol string configuration includes areference symbol string configuration, a first narrowband symbol stringconfiguration and a second narrowband symbol string configuration, thefirst narrowband symbol string configuration having a size in thefrequency axis direction smaller than the reference symbol stringconfiguration, the second narrowband symbol string configuration havinga size in the time axis direction larger than the first narrowbandsymbol string configuration, and the symbol string configurationchanging unit changes the symbol string configuration from the referencesymbol string configuration to the first narrowband symbol stringconfiguration when the receiving state judging unit judges that thereceiving state does not satisfy a predetermined condition, and changesthe symbol string configuration from the first narrowband symbol stringconfiguration to the second narrowband symbol string configuration whenthe receiving state judging unit judges that the receiving state doesnot satisfy the predetermined condition even in the first narrowbandsymbol string configuration.
 6. The radio communication device accordingto claim 3, wherein the radio communication device receives a radiosignal including the symbol string and performs adaptive array controlof the radio signal transmitted to the communication destination, on thebasis of the symbol string included in the received radio signal, thesymbol string includes preamble symbols used in calculation of an arrayweight, and the symbol string configuration changing unit changes thereference symbol string configuration and the narrowband symbol stringconfiguration to symbol string configurations having a same number ofpreamble symbols.
 7. The radio communication device according to claim4, wherein the radio communication device receives a radio signalincluding the symbol string and performs adaptive array control of theradio signal transmitted to the communication destination, on the basisof the symbol string included in the received radio signal, the symbolstring includes preamble symbols used in calculation of an array weight,and the symbol string configuration changing unit changes the referencesymbol string configuration and the wideband symbol string configurationto symbol string configurations having a same number of preamblesymbols.
 8. The radio communication device according to claim 1 or 2,wherein the symbol string includes preamble symbols used in calculationof an array weight, and the symbol string configuration changing unitchanges a symbol string configuration of the preamble symbols defined bya size in the frequency axis direction and in the time axis direction.9. A radio communication method in which a radio signal including asymbol string having a predetermined size in a frequency axis directionand in a time axis direction is received from a communicationdestination by using the orthogonal frequency division multiplexingscheme, and in which an adaptive array control of a radio signal to betransmitted to the communication destination is performed on the basisof the symbol string included in the received radio signal, the radiocommunication method comprising the steps of: judging a receiving stateof the radio signal; and changing a symbol string configuration of thesymbol string defined by the size in the frequency axis direction and inthe time axis direction, on the basis of the judged receiving state. 10.The radio communication method according to claim 9, further comprisingthe step of notification the communication destination of the symbolstring configuration changed in the step of changing the symbol stringconfiguration.
 11. The radio communication method according to claim 9or 10, wherein the symbol string configuration includes a referencesymbol string configuration and a narrowband symbol string configurationhaving a size in the frequency axis direction smaller than the referencesymbol string configuration, and in the step of changing the symbolstring configuration, the symbol string configuration is changed fromthe reference symbol string configuration to the narrowband symbolstring configuration, when it is judged that the receiving statesatisfies a predetermined condition.
 12. The radio communication methodaccording to claim 9 or 10, wherein the symbol string configurationincludes a reference symbol string configuration and a wideband symbolstring configuration having a size in the frequency axis directionlarger than the reference symbol string configuration, and in the stepof changing the symbol string configuration, the symbol stringconfiguration is changed from the reference symbol string configurationto the wideband symbol string configuration, when it is judged that thereceiving state satisfies a predetermined condition.
 13. The radiocommunication method according to claim 11, wherein the symbol stringincludes preamble symbols used in calculation of an array weight, and,in the step of changing the symbol string configuration, the referencesymbol string configuration and the narrowband symbol stringconfiguration are changed into symbol string configurations including asame number of preamble symbols.
 14. The radio communication methodaccording to claim 12, wherein the symbol string includes preamblesymbols used in calculation of an array weight, and in the step ofchanging the symbol string configuration, the reference symbol stringconfiguration and the wideband symbol string configuration are changedinto symbol string configurations including a same number of preamblesymbols.
 15. The radio communication method according to claim 9 or 10,wherein the symbol string includes preamble symbols used in calculationof an array weight; and in the step of changing the symbol stringconfiguration, the symbol string configuration of the preamble symbolsdefined by a size in the frequency axis direction and in the time axisdirection is changed.